I've been using the online recipe calculator for some time now, and I've just recently purchased Beer Tools Pro.
I'm a little confused with the gravity correction under the utilization tab. Not knowing what all the different types of corrections where, I picked "common".
When I did this, my IBU's dropped way lower than what I was used to from the online calculator. If I use no correction, the IBU's are pretty close to each other.

Can someone explain this to me? What does the online calculator use? And what gives me the most accurate IBU's?

slothrob wrote:Ideally you choose the IBU formula that gives the correct IBU's for you system.Remember that individual to individual variability can easily result in actual IBU's that vary 25% or more greater or less than the formula's estimates.

Ok, but what does that really mean? What variables in ones system would make that different from another?

What other factors effect IBUs besides AA%, boil times, boil volume, gravity and final volume. What else is there?

slothrob wrote:Ideally you choose the IBU formula that gives the correct IBU's for you system.Remember that individual to individual variability can easily result in actual IBU's that vary 25% or more greater or less than the formula's estimates.

Ok, but what does that really mean? What variables in ones system would make that different from another?

What other factors effect IBUs besides AA%, boil times, boil volume, gravity and final volume. What else is there?

Good question. I wish I had a good answer. Boil intensity? Kettle construction? If the variables were known or could be controlled they'd probably be included in the formulas.

There is probably a summation of minor differences in technique that add up to these discrepancies. I've seen data from something like a dozen brewers following the same recipe where the measured IBU's averaged very close to the predicted value, but only ~1/3 were within a few IBU's of the target, and just as many were way off.

It's best to taste a lot of commercial beers and pay attention to their IBU's. Then, as you brew your own beer consider whether they taste like you're hitting the predicted IBU using the most appropriate formula, falling high or falling low. Then either adjust your expectations for a given predicted value, try another formula or modify the formula you've already chosen.